Such construction elements may, for example, have the form of sheets or boards. They may be employed in particular in the field of concrete formwork. If they have the form of sheets, they have a thickness of about 12-23 mm, a length in the range of around 60 cm to 6 in and a width of around 20 to 250 cm. These sizes vary from manufacturer to manufacturer. The dimensional details give an idea in which size range the construction elements are used if they are sheets. Broadly speaking, it may be said that length and width vary in order of magnitude in the lower meter range or decimeter range.
If the construction elements are boards, in principle the dimensions are in the same range. Such boards have thicknesses in the centimeter range, lengths in the lower meter range and widths in the lower decimeter range. Boards are used in the construction industry, for example for timber formwork girders, for example H girders. However, they may also be fixed as textured boards in front of the formwork frame. However, they may also be used as boards which make the bottom boards or the side boards on console balustrades.
Sheets are virtually always made up of a plurality of layers such as plywood and provided with a protective film. What is said below with regard to composite formwork sheets generally applies analogously in very many areas to plywood sheets:
1. The sheet expands differently in different directions when heated. PA1 2. If film laminated sheets are subjected to high temperature loads, blistering and detaching effects occur. PA1 3. It is difficult to make the surface repellent to cement paste. The surface loses these repelling properties over time, which is why separate cleaning installations and cleaning machines are necessary. PA1 4. Release agents have to be used. PA1 5. The use of vibrators may cause surface destruction, either by the vibrator touching the sheet directly, or else by the vibrator making the film layer undergo very short oscillations which cannot be absorbed by the bonding joint over long periods and therefore results in detachment. This risk exists in particular in the case of ceiling formwork. PA1 6. The sheets absorb moisture and therefore change their shape after installation. PA1 7. Mechanical damage pierces the film and brings about a loss in load-bearing capacity. PA1 8. The sheets may rot due to decay or fungal attack. PA1 9. The abrasion resistance is not the same over the cross-section, since the material is not homogeneous. PA1 10. Textured sheets have a short service life and are difficult to produce. PA1 11. It is difficult to repair sizeable drill holes in or damage to the formwork skin. PA1 12. The boards are only roughly classified according to load-bearing capacity and modulus of elasticity. They cannot be adequately matched in accordance with use. PA1 13. Recycling is not possible. PA1 14. It is not possible to fix the shape to within tenths of a millimeter. PA1 15. Since the edges are interfaces, they have to be sealed. PA1 16. An elastic joint has to be produced by means of silicone between the nose of the frame leg and the formwork skin. PA1 17. Plastic cones have to be fitted as edge protection. PA1 18. The screws which fix the sheet to the frame cannot be screwed in completely because allowance must be made for the swelling of the sheet. Consequently, the fixing screws initially protrude, which presents difficulties in transportation and impairs the concrete finish. PA1 19. The surface cannot be easily regenerated. PA1 20. The service life is not long enough. PA1 21. The sheets have to be dip-impregnated. PA1 22. Allowance has to be made for the fact that timber will only withstand a certain amount of pressure while the metal supports would allow much higher pressure. PA1 23. Edges can split. In the case of boards in particular, the ends splay. PA1 24. In principle it is impossible to use recycled materials for production. PA1 25. The sheets and boards must be stored protected from the weather. PA1 26. Production is complicated. Modern processes such as compression molding or extrusion are not possible. PA1 27. If the construction element absorbs moisture, the load-bearing capacity fluctuates. PA1 28. The sheets become disproportionately expensive with increasing size of their surface area. However, at the same time there has been a trend for some time to use large-area formwork elements. PA1 29. When fitting the sheets in the frame, the longitudinal and transverse directions have to be considered, since they have different moduli of elasticity even in the case of plied sheets. PA1 30. When fixing sheets having a very high load-bearing capacity, a hole for the screw has to be predrilled in order to prevent the film tearing.